The present invention relates, in general, to safety liners for lenses, and more particularly, to liners for mounting glass or glass/plastic laminated ophthalmic lenses in metal eyeglass frames.
Although glass ophthalmic lens are extremely popular and have many advantageous features, one undesirable characteristic is that when such a lens is formed with a thin peripheral edge and is mounted in the rim portion of a rigid metal frame, the resulting eyeglass product becomes very prone to dangerous lens edge chipping and fracturing. The reason is that whenever glass is ground to a thin and mechanically weakened edge, as occurs with a thinly edged plus-powered lens or a low powered thin glass lens, and then the lens is tightly compressed circumferentially into a metal frame, the hard materials of the lens and frame do not uniformly contact each other around their entire circumferences. As a result, high physical stresses can be induced into the glass at various points of contact around the lens circumference. Such point contact stresses cause dangerous fragility and unpredictable durability, since even a slight impact or a small flexing of the metal frame can often destroy the lens and can cause sharp slivers of glass to be ejected from the lens toward the wearer's eye. On the other hand, if thin glass lenses are inserted into plastic frames, excellent safety performance is achieved as a result of the plastic frame's relatively low hardness and its ability to naturally conform to slight irregularities in the glass edges. However, many eyeglass wearers prefer metal frames, so it is important to make their use as safe as possible.
Glass to plastic laminated ophthalmic lenses have recently been developed, and are disclosed in U.S. Pat. No. 4,679,918. Such lenses have numerous advantages over glass lenses, in that they combine the best properties of both materials so as to produce a light weight but scratch resistant lens. But such laminated composite lenses also suffer the intrinsic and undesirable chipping and fracturing characteristics of the all-glass ophthalmic lens having a thin edge, when the composite lens is mounted in a metal frame. That is, the thin glass layer of the laminated composite lens is still subject to chipping or even fracture because of stresses imposed on the glass portion of the lens by a metal frame.
It has been proposed to overcome the problem encountered with thin-edged glass ophthalmic lenses mounted in metal frame rims by lining the rim groove which receives the lens edge with a soft material. For example, a resilient polyethylene plastic liner has been proposed for metal frame rims in order to simulate the excellent performance obtained with all-plastic frames. Unfortunately, however, the use of frame rim liners has been unsatisfactory. The frame rim liner, it has been found, must be relatively thick in order to simulate the conformity and resilience of an all-plastic frame, but the limited depth of the groove in virtually all metal frame rims presents serious difficulties. Almost all rim groove depths are in the range of 0.5 to 0.8 mm deep, and this standard depth is ordinarily sufficient to safely retain a lens when the peripheral edge of the lens is shaped to match that of the frame rim groove. However, if the rim groove is V-shaped with a maximum depth of 0.75 mm, and a perfectly matched liner of about 0.4 mm thickness is inserted into the V-shaped groove, the liner reduces the effective lens retaining groove depth to about 0.25 mm (or 0.010 inch). The result of this reduction in the effective groove depth is a consequent reduction in the lens retention ability of the frame, and this renders the eyeglass very unsafe because the entire lens can be easily unseated from the frame with only a slight force or impact. Furthermore, a liner of 0.4 mm thickness is insufficient to adequately simulate an all-plastic frame, for the metal frame rim still imparts pressure through such a thin liner to produce stress contact points on the glass when the rim is closed around the lens, and thus fails to provide the safety and durability that is normally expected from an all plastic frame. Thus, even if the frame liner were to provide adequate retention characteristics, it still would not solve the problem of glass fracture.
Accordingly, it is clear that no remedy is presently available for overcoming the problems observed in the mounting of delicate, thin glass lenses in the rims of hard metal frames. Furthermore, these problems are not limited to thin glass lenses, but to glass of all thicknesses when tightly mounted in metal frames without liners, for such lenses are generally less safe and more fragile than their counterparts mounted in plastic frames. Highly skilled opticians in their daily practice routinely tighten the rims of metal frames around glass lenses to a tension just short of the point where the glass will chip. They have learned from years of experience to stop tightening just before damage occurs, but such practices remain dangerous because such tightening of the frame produces stresses which can result in fractures at unpredictable times. A slight frame flexure, a thermal shock, or a mild impact can cause chipping, and even a standard polariscopic inspection for lens stress cannot predict when that might happen. Such mountings are one of the causes of the many thousands of glass lens that are broken each year. Metal frames with thin liners provide small and insufficient improvements in safety, while frames with thicker liners cause unsafe lens retention problems.